Improve Access Control Guidelines

[maxsharabayko]

This is a draft request. Some parts require further clarification!

• Add version schema. Version number format (major.minor or one number) and increment rules (increment on adding new fields?)

• Declare that all single letter key definitions are reserved for the future use. Also instruct users they can add their keys with user_* or companyname_* prefixes. They should not extend existing values. PR [docs] Access control guidelines: reserve single-character keys #1421.

• Tentative. Define and remove future use (unused) definitions. Or put under the corresponding Future Use section. Determine which of the currently defined keys are not used. Mode, type, session ID? Hostname was requested and is probably used.

[nikandfor]

Can I ask why format is called YAML if the content is not parsed by yaml parser? (It actually parsed but as a single string.)